The invention relates to a method for measuring a pressure using a measuring cell arrangement according to the preamble of patent claim 1, as well as to an arrangement for this purpose according to claim 18.
It is known to measure pressures and pressure differences by pressurizing a thin diaphragm and measuring the resulting deflection. A known and suitable method for measuring the deflection of such diaphragms is to design the diaphragm arrangement as variable electric capacitance, wherein via an electronic measuring equipment the capacitance change is evaluated in a known manner, the said capacitance change correlating with the pressure change. The capacitance is formed by positioning the thin, deflectable diaphragm surface slightly spaced to another surface of a body and the two opposing surfaces are formed electrically conductive. When the diaphragm and the body are made of electrically non-conductive, dielectric material, the surfaces are, for example, coated with an electric coating forming the capacitor electrodes. The diaphragm and/or the body themselves can be made of electrically conductive material, wherein the surfaces again form the capacitor electrodes. By pressurizing the diaphragm, the distance between the two electrodes changes due to the deflection, providing an evaluable capacitance change of the arrangement. Sensors of this kind are produced in large quantities, e.g., from silicon. Both, the plane base body and the diaphragm are often completely made of silicon material in this connection. There are also embodiments having combined material compositions, e.g., silicon with a glass base. Thus, the sensors can be produced cost-efficiently. Pressure sensors of this kind are normally only applicable for higher pressure ranges in the range of approximately 10−1 mbar up to several bar. High resolution and lower pressures than 10−1 mbar are not feasible anymore with the material silicon. Sensors of this type are not suitable for typical vacuum applications. For the different vacuum processes to be controlled, measurements of pressures in vacuum are often carried out between atmospheric pressure and 10−6 mbar. Such measurements require a high sensitivity with a high resolution and reproducibility of the vacuum pressure measurement, what only specially designed measuring cells are capable of, which differ completely from the setup of the high pressure-measuring cells.
Capacitive diaphragm pressure-measuring cells being made of corrosion-resistant material, such as Al2O3, are particularly suitable for the vacuum pressure measurement. In EP 1 070 239 B1 (being an integral part of the present application), a capacitive vacuum measuring cell is described, which is substantially built completely of ceramic and is thus corrosion-resistant to a large extent. To be able to measure very low pressures down to 10−6 mbar with a high accuracy, a very thin ceramic diaphragm of a thickness of, e.g., <250 μm is used, which is arranged tensionless and symmetrically inside a ceramic housing. The distance between the capacitor electrodes or between the diaphragm surface and the surface of the housing body, respectively, preferably lies in the range of 2 to 50 μm. The diameters of such diaphragm pressure-measuring cells preferably lie in the range of 5 to 80 mm. The capacitances formed thereby and to be measured for such diaphragm pressure-measuring cells lie in the range of 10 pF to 32 pF. In this connection, the measured capacitive serves as measure for the pressure to be measured. Said capacitance changes according to the pressure dependent deflection of the diaphragm, whereby the pressure being present at the diaphragm can be identified. The capacitance measurement needs to be carried out very precisely and is not entirely easy for such small capacitance values. According to the present state of the art, small interference-prone capacitances are usually digitalized using a sigma-delta capacitance-to-digital converter (CDC). The electronics assembly being necessary for this purpose is arranged on a small printed circuit board behind the diaphragm pressure-measuring cell and connected via a line with the capacitor electrodes of the measuring cell. The digitalized signal is processed and calibrated afterwards within another electronics assembly, which comprises a microprocessor and is arranged on another printed circuit board. Consequently, such a measuring cell arrangement for measuring vacuum comprises a capacitive diaphragm pressure-measuring cell and an electronics assembly for signal processing arranged thereto. Said measuring cell arrangement is connected in the usual manner with the vacuum components to be measured, such as vacuum containers or vacuum pipelines, which contain the gaseous media to be measured.
The measurement of small capacitances is diversely used for reading out the values to be measured of sensors. However, the precise measurement of such small capacitances is not entirely easy. In Baxter's book (Larry K. Baxter, Capacitive Sensors, IEEE Press, NJ 1997) a multitude of possible circuitries is shown. Predominantly, said circuitries were designed such that an as linear as possible correlation to the measurand arises, such that the measurement signal can directly be processed analogously.
The circuitries did not change fundamentally till now, only the opportunity of integration created several interesting additional solutions. For instance, Analog Device has combined a charge weighing machine with a delta-sigma converter within their CDC series of integrated circuitries and developed a very powerful microchip, such as the microchip being named AD7745. A great advantage of said integrated solution is the possibility to be able to reduce problems with the temperature compensation and enable a better shielding of a large part of the critical elements inside the chip housing.
A further option to measure capacitances is to measure the charging time of a capacitor, what in principle can be realized relatively easy. In this connection, the capacitor to be measured is usually charged with a current, e.g., via a constant current source or via a charging resistor. The voltage, the current, and the time needed for the charging are measured and the capacitance is determined therefrom. For small capacitances problems arise in this connection with the measurement of very small currents and the very short measuring times. Capacitances can also be measured in a bridge circuit based on Wheatstone. A popular example for this is the measuring-circuit in which a diode array is used as rectifier. In such a circuitry an amplitude-stabilized sinus source is used. Said sinus sources can be, for instance, an amplitude-stabilized Wien oscillator. Additionally, the comparatively large temperature coefficients of the rectifiers need to be taken into account in this connection. Such a circuitry concept leads to a complex circuit technology.
Capacitive diaphragm pressure-measuring cells are used in vacuum processes for the exact pressure determination. Such vacuum processes comprise a wide variety of processes, such as coating processes, etching processes, thermal treating of workpieces etc. These processes are often operated using supporting gases, which are needed within the process, both, active as reactive gas or also as inert gas. In this connection, the vacuum system is supplied with the gases via a pressure control system or flow control system, respectively. In this case, a capacitive diaphragm pressure-measuring cell can serve as pressure sensor for the control system. For a precise process control it is necessary that the diaphragm pressure-measuring cell measures as precisely as possible, but, in particular, measures fast, too.
Another important application of such diaphragm pressure-measuring cells is also the calibration of high vacuum pressure-measuring cells of a wide variety of designs, such as Pirani type, Penning type, diaphragm pressure-measuring cells etc. In this case, a diaphragm pressure-measuring cell can serve as reference pressure-measuring cell for the comparison with the measuring cells to be calibrated, for example, by means of comparison measurements. An important popular method became also known as static expansion method, which is described more detailed in the book “Wutz, Handbuch Vakuumtechnik” (10. Auflage, Karl Jousten (Hrsg.), Vieweg+Teubner, Wiesbaden 2010, ISBN 978-3-8348-0695-6).
For the application of the capacitive diaphragm pressure-measuring cell as reference pressure-measuring cell in calibration devices, besides the measurement accuracy, in particular, a high measuring speed is also important to be able to record the effective actual pressure values during pressure changes with an accuracy as high as possible. The temperature influences on the measuring accuracy of the reference pressure-measuring cell are also important and need to be controlled as good as possible.
The diaphragm pressure-measuring cell arrangements known by now are slow concerning this matter and the measuring times are larger than 8 ms or even larger than 30 ms. The achievable measuring accuracy of 0.15% to 0.4% of the measured value is often insufficient, too, especially for larger temperature ranges of 5 to 220° C. The measurement range to be measured or the measurable capacitance of the diaphragm pressure-measuring cell, respectively, is limited by the use of known microchips for the measurement electronics (e.g., 21 pF for AD7745). This fact limits the production scope or dramatically minimizes the corresponding yield, respectively, and thus increases the production costs.